It's for when you have really nested directories. It happens especially when you're working in a file space used by others. I used to have a folder I would often reach called /media/nas/documents/personal/school/foo/bar/foobar2001/projectA

I ended up going back to that project so many times, I could just do j projectA and get there from anywhere. "Why not use a symlink?" I hear you say. Well it's because I often have to go to projectB or another which was in another really nested dir. Or I needed to jump to another directory which was equally as nested, and only had to use it frequently for like a week or so. Making and deleting symlinks all the time wasn't practical. Not to mention some software doesn't properly follow symlinks